If a console is banned, it gets recorded in secdata. When this happens, a backup of the original secdata is made in the nand. Use findsecdata to see if there is a backup in the nand.
Otherwise use XVal with your serial number and the number that follows it. You can also goto the US xbox site, register with a random US address, add the console serial number to the profile, submit the console for a repair and if it allows you the console is not banned.
There is also talk of being able to tell from the XBOX live connection test, pressing one of the buttons will come with with status numbers.